Although there are already numerous yellow dyes on the market for colouring plastics, demand still exists for novel dyes with improved properties. In particular, there is a demand for known dyes improved with respect to their fastness. This applies in particular in the case of the use for bulk colouration of polyamide.
The bulk colouration of synthetic polyamides presents higher requirements of the colourants used than the bulk colouration of other plastics. The processing temperatures of synthetic polyamides, particularly in combination with glass fibres, are considerably higher and also the chemical reactivity of molten polyamides, especially of nylon-6.6, is substantially higher such that the heat stability of the colourants used has to be exceptionally good. Pigments generally have high thermal stability. However, there are few pigments which satisfy the high requirements in the case of bulk colouration of plastics, particularly if high light resistance is also additionally required.
Pigments are known from the prior art which are suitable for colouring plastics in shades of yellow.
DE-A 3543512 A1 describes pigments based on azo lakes (Bayplast® yellow G) which may be used for colouring polyamide in shades of yellow.
EP-A 0074515 discloses pigments based on nickel azobarbituric acid complexes which may likewise be used to achieve yellow colouring of polyamide.
Furthermore, long known is the use of Pigment Yellow 192 (C.I. 507300) to achieve yellow colouration of plastic.
Although the pigments mentioned have good thermal stability, no transparent colouration of plastics can be achieved therewith. Pigments can also impair the mechanical properties of the polymers. The use of solvent dyes is known from the prior art in order to colour plastics in transparent shades of yellow. The mechanical properties of polymers are generally not adversely affected by dyes.
Known solvent yellow dyes are e.g. Solvent Yellow 114 (C.I. 47020) from the class of quinophthalone dyes, Solvent Yellow 160:1 (C.I. 55165) from the class of coumarin dyes and also Solvent Yellow 179 (N-2-((4-cyclohexyl)phenoxy)ethyl-N-ethyl-4-(2,2-dicyanoethenyl)-3-methylaniline) and Solvent Yellow 93 (C.I. 48160), both from the class of methine dyes.
The properties of these yellow colourants known from the prior art are not however always sufficient for current existing technical requirements and are in particular in need of improvement regarding their fastness properties, particularly their thermal stability.
Furthermore, yellow methine dyes having good light fastness are known from EP-A 3 048 138, which also represent an improvement with respect to their thermal stability compared to the prior art presented above, but are nevertheless worthy of further improvement since the performance requirements in terms of polyamide colouration have increased still further.